An optical communication system employing an optical transmission line such as an optical fiber simultaneously transmits a plurality of optical signals each having a different wavelength with an optical fiber through WDM (wavelength division multiplexing). The optical communication system employs a bidirectional optical transmission module in a transmission terminal and a receiving terminal for optical signals. The bidirectional optical transmission module of this kind has become publicly known in, for example, the Patent Document 1: U.S Pat. No. 5,555,334. In the bidirectional optical transmission module described in the Patent Document 1, light-emitting element 100 and light-receiving element 700 are housed in same package 120 as shown in FIG. 7, and holographic diffraction grating 600 is provided on the upper or lower surface of cover glass 220 that is located at an opening of the package.
For transmitting signals to the bidirectional optical transmission module, a light beam emitted from light-emitting element 100 passes through the diffraction grating 600, and a 0th order diffracted light beam is converged by lens 300 on surface 500 on an end of optical fiber 400. For receiving signals, a light beam emitted from the surface 500 on the end of optical fiber 400 arrives at diffraction grating 600 through lens 300. Then, the light beam is diffracted by the diffraction grating 600, and +1st order diffracted light beam is converged on a light detection surface of light-receiving element 700 so that transmitted signals transmitted through optical fiber 400 are received.
The diffraction grating 600 is constructed to be of a two-level grating in which linear grating grooves 200 are formed on a flat surface of cover glass 220 (refractive index: n), as shown in FIG. 8. However, if the two-level grating is used for the diffraction grating 600, respective dimensions of groove depth d, groove width W and pitch of groove P of the grating structure are uniquely determined on the maximum condition of the 1st diffracted light with wavelength λ1 given by λ1/(2 (n−1)), resulting in a problem that it is difficult to improve diffraction efficiency of 0th order diffracted light with another wavelength λ0.